Metallic carbides used as microphone resistance granules



Patented Jan. 29, 1952 METALLIC CARBIDES USED AS MICRO- PHONE RESISTANCE GRANULES Eric Arvid Ericsson, Stockholm, and Anders Ossian Jiirgensen, Traneberg, Sweden, assignors to Telefonaktiebolaget L M Ericsson, Stockholm, Sweden, a company of Sweden No Drawing. Application May 8, 1950, .Serial No.

7 160,808. In Sweden May 25, 1949 This invention relates to microphones containing metal carbides in granular form as the pressure responsive elements thereof.

It is a well-known desire, that a. microphone which is to transform variations of the pressure in air, water or another medium into electrical current variations should have a suflicient sensitivity, that is, give" rise tog'reat variations of the resistance withsmalrvariauons of the pressure. The relative variations of the contact resistance must chiefly be proportional to the relative variations of the pressure. At the same time such a static contact resistance is desired, as can through suitable treatment be varied within certain limit at the same volume of granules in the transmitter inset. A relatively great static resistance is especially valuable. A more favourable matching of the static resistance of the transmitter to the input impedance of the connected telephone line is thereby obtained. A high resistance also requires a lower current and consequently a lower current consumption. The granulated material in the transmitter should also be chemically and physically constant.

These requirements are not met with to a sufficient degree in the granulated materials for transmitters known until now.

A condition for obtaining a high sensitivity and good qualities in other respects is that the granulated material has a low external resistance within each granule in relation to the contact resistance between the different granules. It has been tried to achieve this by coating metal granules with a layer of poor conducting material. These experiments have however not given any practlcal results.

According to this invention a material is used,

3 Claims. (01. 179'190) chosen, for example a semi-conductor of the excessor the defect conductor type. An example of a semi-conductor of the excess conductor type is the material obtained from SnOz by reduction with hydrogen gas, whereby oxygen atoms are removed and there arise divergences in the stoichiometric composition of SnOz. This reduction (which in certain cases can also aim at a' red'u'ction'of a nietalloid component) entails a'i liberation of metal valences, which considerably" increases the conductivity. The condition is the inverse for defect conductors (also called oxidationor hole conductors). We take for example CuzO with a conductivity 210- 1/0 cm. and obtain by oxidation with oxygen a defect conductor Conductivity at great divergence from stoichiometric composition (after reduction) Conductivity at near stoichiometric composition Some typical examples of defect conductors the external resistance of which does not differ are:

Initial o d r it i Ti 0 a t 011 IV y 110 018- 61' 011 110 IV] y matenal oxidation considerably from the external resistance of the granule, but the contact resistance of which can be made considerably higher, thereby allowing greater variations at changes in the contact pressure. According to the invention, this is achieved by making use of the known fact, that the contact resistance of a certain material grows with the hardness. A material with great hardness is It has appeared that excess and respectively defect conductors with two or more free valences are considerably more stable chemically and physically than conductors with one free valence.

Especially good results have been obtained with SnOz, which is found in nature in the shape of cassiterite as large crystals or small granules, mixed with sand. Its colour varies from diamond shining clear yellow to carb n e i h tempe a ur azssavsa V percentage and the nature of the impurities. The

black kind, containing iron as the most probable impurity, shows a hardness up to 70.000 ken/cm This material is practically an insulator in spite of the impurities.

V For the. production of excess conductors in the shape of granulated material it is possible to proceed from cassiterite, which is crushed to the desired granule size. It is then reduced in a current of hydrogen gas at a certain tempera-. ture, for example between 500-700 ,C.-and for a certain time, for example one hour. An excess conductor having two free metal valences for each reduced oxygen atom is thereby obtained.

Unreduced SnOz can however occur in the core v have also been Proved. .They have been produced from waterfre'ej (that is glowed) oxides with The low st 1 0 sible temperature was for vanadium carbide (VC) 11-00 C. and for tantalum carbide (TaC) 1250 C.

These two carbides have shown the greatest changes of resistance at certain changes of the Pressure.

4. A granulated material with varying composition, contact resistance and hardness can be obtained by reduction and respectively oxidation in difierent manners at different temperatures, pres-' .dependent electrical resistance, granules of a metal carbide.

2. A microphone containing, as the pressuredependent electrical resistance, granules of vanadium carbide.

3. Armicrophone containing, as the pressuredependent electrical resistance, granules of tantalum carbide.

. ERIe ARVID ERIcsspN.

ANDERS OSSIAN JORGENSEN.

REFERENCES CITED The lo n r ferences are of record in the file of this patent;

UNITED STATES PATENTS Chr st s n. "we... Mar; 2 .1. 19" 

1. A MICROPHONE CONTAINING AS THE PRESSUREDEPENDENT ELECTRICAL RESISTANCE, GRANULES OF A METAL CARBIDE. 